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Toxoplasmosis is a disease caused by infection with an organism called Toxoplasma gondii, a microscopic single cell organism which is one of the most common parasites in animals.  Although virtually all warm-blooded animals and humans can be infected with this organism, it is an extremely well adapted parasite and rarely causes significant disease to individuals which it infects.

How is it transmitted?

Cats are usually infected by eating the organism present in the tissues of another infected animal, normally a rodent.  The organism replicates locally in the intestine of the cat and also replicates within the body.  The replication in the intestine results in shedding of eggs (oocysts) in the faeces.  However, an immune response rapidly develops which halts both shedding of eggs and replication of the organism in the body.  Despite the immune response, infection still persists in the form of microscopic cysts present in some tissues of the body, although this doesn’t usually result in any disease.  The oocysts shed in the faeces are very resistant and can contaminate the environment for several years.  Other animals become infected by eating these eggs.  Cats are the only animal in which the Toxoplasma gondii replicates in the intestine, resulting in the shedding of eggs in faeces therefore cats are essential to the life cycle of the organism.

Humans can be infected from ingesting oocysts from soil (through gardening) but a more common way of humans becoming infected is through the food chain.  Sheep, cattle and pigs grazing on contaminated pastures, or fed oocyst-contaminated food can also develop the encysted form of the organism in body tissues.  If infected meat is not cooked adequately enough, or if poor hygiene precautions are adopted during the handling of uncooked meat, humans can become infected.

What disease does Toxoplasma cause in cats?

Although Toxoplasma is a relatively common infection, it usually causes no disease in infected cats.  Rarely cats fail to develop an adequate immune response to the organism which may allow it to continue to replicate and cause damage to tissues.  When this happens a variety of different clinical signs can develop including eye disease, respiratory disease, diarrhoea, liver problems and nervous signs.  Toxoplasma is a rare cause of disease.

How common is it?

The proportion of cats infected with Toxoplasmosis varies according to their lifestyle.  Due to the way in which the organism is transmitted, infection is more common in a stray, feral, farm cats and others which engage in a lot of hunting or are fed raw meat.  As many as 60% of these cats are infected with Toxoplasma.  In contrast, infection is uncommon in pet cats who do little or no hunting and which are fed exclusively on commercial cat foods.

How do you diagnose and treat Toxoplasmosis?

Toxoplasmosis is difficult to diagnose in cats.  Blood tests are available to show whether a cat has been exposed to the organism, but these tests do not necessarily mean that Toxoplasma is the cause of any disease showing.

An appropriate antibiotic is used to treat suspected cases of toxoplasmosis.

How are humans affected by Toxoplasmosis?

Around a third of the UK population has been exposed to toxoplasmosis.  As with cats, the majority of people infected experience no clinical signs at all. 

However, as with cats, there are also some individuals where disease can occur and at particular risk are pregnant women.  If a woman acquires Toxoplasmosis during pregnancy, the infection may be transmitted to the foetus, sometimes causing severe damage.  However, this is only a risk if the woman acquires the disease for the first time.  A woman who has previously been exposed to the organism carries no risk of transmission to a foetus if she subsequently becomes pregnant.

Following sensible environmental and meat hygiene measures can greatly reduce the risk of human infection.

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